When load current flows from an AC power supply to a load, a so-called harmonic component is typically generated in the load current. The harmonic component is a well-known problem as it causes so-called harmonic interference, and is thus to be reduced.
An active filter is proposed as one approach to solving the problem. Particularly a parallel active filter is connected to the AC power supply through an interconnection reactor and passing compensating current to reduce a harmonic component of power current flowing through the AC power supply.
Specifically, in order to reduce the harmonic component of the power current, processing of passing compensating current of an opposite phase to this through the parallel active filter (or supplying compensating current of the same phase as this from the parallel active filter) is performed.
The harmonic component of the load current is adopted as a command value of the compensating current, and, based on a deviation of the compensating current from the command value, a current controller operates to control operation of the parallel active filter.
The parallel active filter is introduced, for example, in Katsuhiro Izumi and four others, “Influence of the Compensation Current Detection Characteristic on the Active Filter Performance”, Reports of the Faculty of Engineering, Nagasaki University, Vol. 30, No. 55, pp. 165-170, July 2000 and FIG. 10 of Japanese Patent No. 3755220.
Further, Japanese Patent Application Laid-open No. 5-252751 and Japanese Patent Application Laid-open No. 2008-234298 are listed as prior art documents disclosing technology related to the present application. Japanese Patent Application Laid-open No. 5-252751 discloses technology for suppressing overcurrent in the event of accidents, and Japanese Patent Application Laid-open No. 2008-234298 discloses technology for suppressing a time delay of operation of performing rotating coordinate conversion and a delay occurring when current is detected.